1. Field of the Invention
The present invention relates to a color thermal printer having plural thermal heads for respective colors, which prints plural full-color images on a continuous recording paper.
2. Description Related to the Prior Art
A thermal printer, such as a thermosensitive color priner, is well known, in which a thermal head heats a thermosensitive recording paper having a thermosensitive coloring layer to record a mono-color image. The thermal head has plural heat elements arranged in a main-scanning direction. In the thermal printer, the thermosensitive recording paper is transported in a sub-scanning direction relatively to the thermal head, and thereby a mono-color image is recorded line by line. Each heat element is driven on the basis of image data of pixel for each line, to apply a print energy (or heat energy) to the thermosensitive recording paper.
A friction coefficient between the thermosensitive recording paper and the thermal head varies in accordance with the total heat energy for one line. When the total heat energy becomes higher, then the temperature of the heat elements increases, and therefore the friction coefficient becomes smaller. When the total heat energy becomes lower, then the temperature of the heat elements decreases and therefore the friction coefficient becomes larger. Accordingly, when the color density of the original image varies suddenly, then the variation of the heat energy is so large that the friction coefficient varies suddenly. When the friction coefficient varies, then the transport friction between the thermosensitive recording paper and the thermal head varies, and therefore the transport speed of the thermosensitive recording paper varies. Accordingly, the heat energy applied to a unit area varies, which causes the printed mono-color image to have unevenness.
The Japanese Patent Laid-Open Publication No. 2002-67370, for example, discloses a thermal printer, in which the mono-color image is printed without unevenness although the transport friction varies in accordance with the heat energy. In the thermal printer, the transport friction of the thermal head is calculated at each line, and a transport load fluctuation, or a transport friction fluctuation, between a recording line and previous recorded line is calculated from each transport friction. Then, image data for the recording line of the mono-color image is corrected on the basis of the transport friction fluctuation. Thus, although unstableness of the transport friction fluctuates the transport speed, the heat energy for each heat element is adjusted in accordance with the change of the transport speed. Therefore it is prevented to generate the unevenness in the printed mono-color image.
By the way, a color thermal printer is known. In the color thermal printer is used a color thermosensitive recording paper having a cyan, a magenta, and a yellow coloring layer on the base in this order. The color thermal printer prints a full-color image on the color thermosensitive recording paper. There is a type of the color thermal printer, in which the continuous color thermosensitive recording paper is transported to form the full-color images sequentially in plural print areas. In a transport path, yellow, magenta and cyan thermal heads are disposed in this order, and yellow, magenta and cyan images are simultaneously printed in the different print areas with the respective yellow, magenta and cyan thermal heads. Thus the plural full-color images are formed sequentially.
However, in the color thermal printer, when the above-described adjusting method of the heat energy is applied to each thermal head, the occurence of unevenness of each mono-color image in the variation of the transport friction cannot be prevented. In the color thermal printer, the transport friction fluctuation at the one thermal head has influences on the printing of the each mono-color image with other thermal head. For example, when the transport friction varies at the yellow thermal head, then the transport speed of the color thermosensitive recording paper varies. Accordingly, not only the yellow image but also the magenta and cyan images have unevenness.
An object of the present invention is to provide a color thermal printer in which it is prevented that variation of a friction of a thermal head causes unevenness of coloring when plural mono-color images are recorded with thermal heads.
In order to achieve the object and the other object, a color thermal printer of the present invention includes first-Nth thermal heads having plural heat elements, first-Nth platen rollers confronting to the first-Nth thermal heads respectively, and a calculation means. Each of the heat elements arranged in a main-scanning direction is driven on a basis of a corrected mono-color image data to apply a heat energy for coloring to a coloring layer of a color thermosensitive recording paper. The calculation means calculates a friction between each thermal head and the color thermosensitive recording paper on the basis of the heat energy corresponding to a mono-color image data at each thermal head. Further, the calculation means calculates a transport friction fluctuation of the frictions between the recording line and a first previous line. Then the mono-color image data of the Kth thermal head is adjust on the basis of all of the transport friction fluctuation of the first-Nth thermal heads to become the corrected mono-color image data, while K satisfies 1xe2x89xa6Kxe2x89xa6N.
In the color thermal printer, the calculation means may be provided with first-Nth calculation sections for calculating the transport friction fluctuation of the first-Nth thermal heads. The first-Nth calculation section further has first-Nth memories for memorizing the transport friction fluctuation of the first-Nth thermal heads respectively.
Further, in the thermal printer, the number N is three, and the first, second, and third thermal heads are yellow, magenta and cyan thermal heads. When the yellow thermal head starts one line recording earlier by P (0xe2x89xa6Pxe2x89xa61) line than the magenta thermal head, then the transport friction fluctuation FY(M) of the yellow thermal head satisfies a following formula:
FY(M)=Fy(M)+Pxc3x97(Fm(Mxe2x88x921))+(1xe2x88x92P)xc3x97Fm(M).
In the formula, Fy(M) is a transport friction fluctuation by a yellow thermal head when Mth line recording is made. Fm(Mxe2x88x921) is a transport friction fluctuation by a magenta thermal head when (Mxe2x88x921)th line recording is made. Fm(M) is a transport friction fluctuation by a magenta thermal head when Mth line recording is made.
According to the color thermal printer of the present invention, the first-Nth thermal heads are driven to record respective mono-color images on continuous color recording sheet almost at the same time and to print full-color images sequentially. The calculation means provided in the thermal printer calculates a transport friction fluctuation at each thermal head, and the heat energy of each thermal head is adjusted on the basis of all of the transport friction fluctuations. Accordingly, it is prevented that the unstableness of the friction of each thermal head causes the unevenness in the full-color image.